1. Field of the Invention
The present invention relates generally to measuring acceleration using optical measurement by light interference, and more specifically to an improved fiber optic accelerometer for measuring acceleration by sensing acoustic waves.
2. Description of Related Art
The flexure or strain of a flexible disk or membrane is a well-known basis for measurements including acceleration and pressure, acceleration typically being measured by such flexure resulting from momentum forces acting on such a disk in a direction along its axis. The amount of flexure may be determined interferometrically, where interferometric measurements of strain can provide great resolution and, when carried out with an optical fiber, provide a simple and rugged sensor which requires low power, is immune to electromagnetic interference, and is adapted to remote sensing and to high data rates.
It is known to minimize operating condition errors by using a xe2x80x9cpush-pullxe2x80x9d arrangement of a pair of complementary interferometer optical fiber legs such that a change in a measured variable shortens one leg and respectively lengthens the other leg. Light signals traveling through both legs are interfered with one another, where the corresponding amount of interference between the light signals is indicative of the change in length of each of the legs. In order to further increase the sensitivity of a fiber optic sensor legs, accelerometers have been developed where the optical fiber 10 is wound into a spiral 12 and affixed to a side of a membrane 14, as shown in FIG. 1.
Further accelerometer designs have attached such fiber optic wound coils to both sides of the deformable membrane. Each optical fiber coil serves as one leg of an interferometer. Referring to FIG. 2, in these sensor designs, each side of an elastic and circumferentially-supported or axially-supported membrane 20 is spirally wound with coils 22a, 22b of optical fiber fixedly attached to the membrane sides 24a, 24b. As a result of this construction, flexure of the membrane expands such a coil on one side of the membrane and contracts such a coil on an oppositely facing side. The membrane disk 20 acts as a type of a spring which is deformed by inertial force resulting from acceleration due to an external force, such as an acoustic wave traveling in a liquid. The membrane is mounted on a body so that an acoustic pressure differential to be measured exists across the membrane, the spiral coils then being connected for push-pull operation as two legs of a fiber optic interferometer to provide an output corresponding to the flexure resulting from acceleration.
In such conventional accelerometers utilizing wound fiber optic coils, the optical fiber is wound in a spiral fashion starting with coils having a rather small diameter at the center of the membrane with a progressively increasing diameter toward the membrane periphery. This arrangement presents several significant difficulties with its use. Initially, the disparity in the diameter of the windings of the optical fiber coil make it difficult to select an appropriate grade of optical fiber. The smaller diameter windings of the coil near the center of the membrane cause light leak when the bend radius is smaller than a particular optical fiber specification. This constrains the design geometry as well as limits the optical fiber selection to specialty fibers having small bend diameter specifications, which also tend to have a prohibitively high cost for many applications.
A second difficulty associated with use of a wound fiber optic coil comes from the way the optical fiber is spirally wound and attached to the membrane, such that the inner windings and the outer windings of the fiber spiral are deformed differently in response to the membrane deformation. For example, in the case of a peripherally supported membrane attached to a rigid housing, the unrestrained center portion of the membrane will deform more than the periphery of the membrane from inertial lag in response to the external force. This causes the outer windings of the coils to deform only slightly so that they do not contribute much to the overall optical path difference, thus making inefficient use of the optical fiber.
A third difficulty resides in the manufacturing of the fiber optic coil. The winding of an optical fiber to produce a coil of this geometry is a complex procedure, where an uneven wind may result even when the utmost of care is taken. There is also an associated loss for the optical fiber each time it crosses over the adjacent windings of other layer of windings in the fiber optic coil. Further, the wound coil in this design requires its subsequent direct attachment to the membrane, whose inherent stiffness absorbs a greater portion of the flexural strain, thereby reducing the available strain energy that could otherwise be directly transferred to the fiber causing a further loss of efficiency and corresponding reduction in sensor sensitivity.
The foregoing shortcomings and disadvantages of the prior art are alleviated by the present invention that provides a fiber optic accelerometer having a pair of fiber optic coils positioned around a deformable support structure. The support structure possesses a nominally cylindrical shape with the fiber optic coils being wound around opposite ends of the cylindrical support structure from each other. The support structure deforms from its nominally cylindrical shape to a conical shape in response to acceleration along a sensing axis. The changing shape of the support structure causes one of the fiber optic coils to expand while the other of the fiber optic coils contracts. The fiber optic coils are included in an interferometer such that acceleration along the sensing axis produces a phase difference between light signals propagating in the fiber optic coils resulting from their expansion and contraction.